1. Field of the Invention
The present invention relates generally to a method and apparatus for repairing a submerged pipeline. In particular, the invention relates to a method and apparatus for replacing a damaged section of a submerged pipeline without a significant loss of product from or entry of seawater into the undamaged pipeline.
2. Description of the Related Art
Thousands of miles of pipeline lay on the seabed. Modern pipe laying technology permits the installation of pipelines in a variety of water depths, including water depths of thousands of feet. A serious problem results when damage occurs to a pipeline set in deep water because repair can be cumbersome and difficult. Typically. submerged pipelines are damaged by an object impacting it, by an anchor dragging across it, or by the environment corroding it. If a section of pipeline becomes significantly damaged, it may requirement replacement.
In the past, generally two methods were employed to repair a damaged submerged pipeline. One method required the damaged section of the pipeline to be recovered to the water surface for repair on the deck of a large surface vessel. After the repair was completed, the pipeline was lowered and repositioned on the seabed. A disadvantage of this method included the expense associated with recovering the pipeline to the surface. Another disadvantage with this method was that it typically could only be used with small diameter pipelines in relatively shallow water.
A second method for repairing a damaged pipeline involved cutting the pipeline into two pieces near the damaged section while the pipeline remained on the seabed. The cut end of each pipe section was pulled to the surface. Once at the surface, the damaged section of pipeline was replaced with a new section. Specially designed connectors gripped the pipe at the cut ends and provided a flange or collet hub that could be used to connect the new section to the pipeline. Often, a third connector was used to make a telescoping spool section that could be expanded between the two cut ends of the pipeline to facilitate the connection. A disadvantage of this method was that product could escape from the pipeline to the subsea environment and seawater could enter into the pipeline.
To prevent product from escaping into the environment from the cut pipeline, hot tapping systems were used to plug the pipeline. In general, hot tapping systems use a clamp-on, split tee to provide a fluid tight connection in the pipeline. A special drill was attached to the tee in a manner that prevents loss of pipeline fluid during the tapping operation. The tapping drill was fitted with a special cutter that opened a full diameter hole in the side of the pipeline. After cutting a hole in the pipeline, a stopper machine replaced the drill. The stopper machine installed an elastomeric stopper into the pipeline. After the new pipeline section was attached to the remaining pipeline, the stopper was removed but the tee became a permanent part of the pipeline.
A disadvantage of the hot tapping and stopper method was that although the hot tap tee typically could seal under fill pipeline pressure, the tee needed to provide a structural tension capacity to make up for the removal of half of the pipe wall, which was cut away during installation of the stopper. The need for structural capacity demanded that a mechanical connection be made to the pipeline which could withstand full line pressure.
Another disadvantage of the hot tapping system was that the opening in the side of the pipe presented a hazard to the passage of a pipeline pig. The opening needed to be protected with bars or plates such that a pipeline pig did not catch on the opening. Eliminating this hazard was typically cumbersome and complicated in deepwater where repairs are usually performed by an ROV.